Foam mattress with progressive support characteristics and method for manufacturing the same

ABSTRACT

A mattress includes top and bottom foam layers. A bottom surface of the top foam layer and a top surface of the bottom foam layer include protrusions and recessions. The protrusions of the top surface of the bottom foam layer protrude into the recessions of the bottom surface of the top foam layer, and the protrusions of the bottom surface of the top foam layer protrude into the recessions of the top surface of the bottom foam layer. The most protruded surfaces of the protrusions of the bottom surface of the top foam layer are separated from corresponding surfaces of the recessions of the top surface of the bottom foam layer by a channel, but there is no channel between most protruded surfaces of the protrusions of the top surface of the bottom foam layer and corresponding surfaces of the recessions of the bottom surface of the top foam layer.

TECHNICAL FIELD

Illustrative embodiments relate to mattresses, and in particular, foam mattresses with progressive support characteristics, improved airflow and cooling properties, and methods for manufacturing the same.

BACKGROUND

Mattresses contain some type of support element, such as an innerspring core or a foam core made of polyurethane form, for example.

Some people prefer foam core mattresses to innerspring core mattresses. However, foam core mattresses have several drawbacks as compared to innerspring core mattresses.

For instance, unlike conventional foam core mattresses, innerspring core mattresses can deliver support to a person through the resistance provided by innersprings. Such innersprings may be configured to hold the person in a proper postural alignment, while evenly redistributing the person's body weight across a wide area so as to relieve interface pressure.

Further, some innerspring core mattresses are configured to provide progressive support to a person—meaning that the innerspring core provides progressively increasing firmness as more weight is applied to the innerspring core. Such progressive support innerspring cores can provide increased comfort while maintaining proper postural alignment. However, such progressive support characteristics have not yet been reliably attained using conventional foam core mattresses.

Foam core mattresses also have a drawback in that polyurethane foams are good heat insulators. Thus, mattresses with a polyurethane foam support core or a polyurethane foam topping can retain too much heat and cause discomfort for some people as they sleep on the mattress. Indeed, many people complain of becoming too warm while sleeping on a foam mattress or foam mattress topping.

Additionally, conventional methods of manufacturing foam mattresses, such as die-cutting processes and contour-cutting processes are time-consuming and wasteful. Thus, new manufacturing methods are desired for manufacturing foam mattresses that reduce and/or eliminate wasted material and which reduce manufacturing time.

The above have been long felt, but unsolved problems in the bedding industry.

SUMMARY

Illustrative embodiments overcome the above disadvantages and other disadvantages not described above. Also, illustrative embodiments are not required to overcome the disadvantages described above, and an illustrative embodiment may not overcome any of the problems described above.

According to one illustrative embodiment, a mattress comprises: a top foam layer; and a bottom foam layer, wherein a bottom surface of the top foam layer comprises a series of protrusions and recessions, wherein a top surface of the bottom foam layer comprises a series of protrusions and recessions, wherein the protrusions of the top surface of the bottom foam layer protrude into the recessions of the bottom surface of the top foam layer, wherein the protrusions of the bottom surface of the top foam layer protrude into the recessions of the top surface of the bottom foam layer, wherein most protruded surfaces of the protrusions of the bottom surface of the top foam layer are separated from corresponding surfaces of the recessions of the top surface of the bottom foam layer by a channel, and wherein there is no channel between most protruded surfaces of the protrusions of the top surface of the bottom foam layer and corresponding surfaces of the recessions of the bottom surface of the top foam layer.

According to another illustrative embodiment, a method for manufacturing a mattress is provided, the method comprising: cutting a first foam member in half to form first and second top layers, a bottom surface of the first top layer comprising a series of protrusions and recessions and a top surface of the second top layer comprising a series of protrusions and recessions opposite to the series of protrusions and recessions of the first top layer; cutting a second foam member in half to form first and second bottom layers, a bottom surface of the first bottom layer comprising a series of protrusions and recessions and a top surface of the second bottom layer comprising a series of protrusions and recessions opposite to the series of protrusions and recessions of the first bottom layer; and positioning the first top layer on top of the second bottom layer such that the protrusions of the top surface of the second bottom layer protrude into the recessions of the bottom surface of the first top layer and the protrusions of the bottom surface of the first top layer protrude into the recessions of the top surface of the second bottom layer; wherein the cutting the first foam member in half and the cutting the second foam member in half are performed such that, when the first top layer is positioned on top of the second bottom layer, most protruded surfaces of the protrusions of the bottom surface of the first top layer are separated from corresponding surfaces of the recessions of the top surface of the second bottom layer by a channel, and there is no channel between most protruded surfaces of the protrusions of the top surface of the second bottom layer and corresponding surfaces of the recessions of the bottom surface of the first top layer.

BRIEF DESCRIPTION OF THE FIGURES

Illustrative embodiments are described below by reference with the following figures, in which like numerals indicate like elements.

FIG. 1 illustrates a cross-sectional view of top layers for progressive foam mattresses consistent with an illustrative embodiment;

FIG. 2 illustrates a cross-sectional view of bottom layers for progressive foam mattresses consistent with an illustrative embodiment;

FIG. 3A illustrates a cross-sectional view of a progressive foam core comprising a top layer and a bottom layer consistent with an illustrative embodiment;

FIG. 3B illustrates an enlarged view of a portion of the progressive foam core comprising a top layer and a bottom layer, just before the top layer and the bottom layer are fitted together consistent with an illustrative embodiment;

FIG. 4 illustrates a cross-sectional view of a typical build-up configuration of a progressive foam mattress 400 consistent with an illustrative embodiment;

FIG. 5 illustrates a cross-sectional view of a firm build-up configuration of a progressive foam mattress 500 consistent with an illustrative embodiment;

FIG. 6 illustrates a top view of the bottom layers 21 and 22 consistent with the illustrative embodiment shown in FIG. 2, after the bottom layers 21 and 22 have been separated and positioned with the protrusions and recessions thereof facing upward; and

FIG. 7 illustrates a flow chart describing a method of manufacturing a progressive foam mattress consistent with an illustrative embodiment.

DETAILED DESCRIPTION

FIG. 1 illustrates a cross-sectional view of top layers 11 and 12 for a progressive foam mattress consistent with an illustrative embodiment. As shown in FIG. 1, a first foam member 100 is cut in half to form two separate top layers (i.e., a first top layer 11 and a second top layer 12) that may be used in two different progressive foam mattresses, respectively. Such cutting may be performed with any suitable means for cutting including, but not limited to, a blade, laser, a convoluter machine, etc. According to an illustrative embodiment, the foam may be polyurethane foam, or the like.

As shown in FIG. 1, the foam member 100 is split in half so that the first top layer 11 and the second top layer 12 are each convoluted and comprise a series of protrusions and recessions. Specifically, the foam member 100 is cut in such a way that the series of protrusions 16 and recessions 14 of the first top layer 11 is formed simultaneously with the series of protrusions 13 and recessions 15 of the second top layer 12.

That is, for example, as shown in FIG. 1, as the foam member 100 is split by a cutting action, such cutting simultaneously forms a recession 14 in the first top layer 11 and a corresponding opposite protrusion 13 in the second top layer 12. Such cutting continues along the entire length of the foam member 100 until the first top layer 11 is separated from the second top layer 12. Conventional techniques for splitting a foam using a convoluter machine, as are known in the art, can be used to split foam member 100.

Thus, consistent with an illustrative embodiment, two separate top layers (i.e., a first top layer 11 and a second top layer 12) that may be used in two different progressive foam mattresses, respectively, are formed in a single spitting operation which requires approximately 20-30 seconds of machine time. In contrast, conventional die-cutting processes and contour-cutting processes for manufacturing foam cores for mattresses typically require 10-15 minutes of machine time to form comparable foam layers.

In further contrast to conventional die-cutting processes and contour-cutting processes for manufacturing foam cores for mattresses, virtually no wasted foam material results in cutting the foam member 100 in half consistent with an illustrative embodiment.

According to an illustrative embodiment, the top layers 11 and 12 may have a plurality of holes or channels 17 that extend from the top surface of the top layers 11 and 12 to the bottom surface of the top layers 11 and 12, such as described in U.S. Patent Application No. 61/425,303 filed Jan. 22, 2011, entitled Mattress with Improved Cooling Properties, which is incorporated herein by reference. The holes 17 facilitate airflow in a height direction of the top layers 11 and 12. The dimensions and/or configuration of the holes 17 can be adjusted to control and reduce the heat insulating properties of the top layers 11 and 12 to thereby provide increased comfort to a person resting thereon.

According to an illustrative embodiment, the holes 17 may have a diameter of ¼ inch, for example. In one embodiment, the holes 17 in the top layers 11 and 12 can be formed in a grid pattern and spaced apart a distance of 2 inches by 2 inches.

It will be understood that forming holes 17 in the top layers 11 and 12 can reduce the structural support of the top layers 11 and 12. Accordingly, only as many holes 17 as are needed to provide the desired amount of airflow may be formed in the top layers 11 and 12, so as to retain the structural support of the foam. Although the holes 17 in one illustrative embodiment are ¼ inch in diameter, other size holes may be used depending on the desired airflow and support characteristics for the top layers 11 and 12.

Similar to the top layers 11 and 12 of FIG. 1, FIG. 2 illustrates a cross-sectional view of bottom layers 21 and 22 for a progressive foam mattress consistent with an illustrative embodiment. As shown in FIG. 2, a second foam member 200 is split in half to form two separate bottom layers (i.e., a first bottom layer 21 and a second bottom layer 22) that may be used in two different progressive foam mattresses, respectively.

As shown in FIG. 2, the foam member 200 is split in half, using a convoluter machine for example, so that the first bottom layer 21 and the second bottom layer 22 are each convoluted and comprise a series of protrusions and recessions. Specifically, the foam member 200 is split in such a way that the series of protrusions 26 and recessions 24 of the first bottom layer 21 is formed simultaneously with the series of protrusions 23 and recessions 25 of the second bottom layer 22.

That is, for example, as shown in FIG. 2, as the foam member 200 is split by a cutting action, such cutting simultaneously forms a recession 24 in the first bottom layer 21 and a corresponding opposite protrusion 23 in the second bottom layer 22. Such cutting continues along the entire length of the foam member 200 until the first bottom layer 21 is separated from the second bottom layer 22.

By way of illustration, FIG. 6 illustrates a top view of the bottom layers 21 and 22 consistent with the illustrative embodiment shown in FIG. 2, after the bottom layers 21 and 22 have been separated and positioned with the protrusions and recessions thereof facing upward.

After forming the top layers 11 and 12 and the bottom layers 21 and 22, the top layer 11 is positioned on top of the bottom layer 22, as shown in FIG. 3A, to form a progressive foam core 300 for a progressive foam mattress consistent with an illustrative embodiment. As shown in FIG. 3A, the protrusions 23 of the bottom layer 22 protrude into the recessions 14 of the top layer 11, fitting together like pieces in a jigsaw puzzle. Likewise, the protrusions 16 of the top layer 11 protrude into the recessions 25 of the bottom layer 22.

FIG. 3B illustrates an enlarged view of a portion of the progressive foam core 300 for a progressive foam mattress comprising a top layer 11 and a bottom layer 22, just before the top layer 11 and the bottom layer 22 are fitted together consistent with an illustrative embodiment.

Significantly, the top layer 11 and the bottom layer 22 are configured such that, after the top layer 11 is positioned on top of the bottom layer 22, as shown in FIG. 3A, a void space remains between the protrusions 16 of the top layer 11 and the recessions 25 of the bottom layer 22, thereby forming a channel 30 through which air may flow. In particular, the most protruded surfaces of the protrusions 16 of the top layer 11 are separated from the corresponding surfaces of the recessions 25 of the bottom layer 22 by the channel 30.

Such a configuration (among other advantages) provides the progressive foam core 300 with progressively firmer support characteristics as force applied to a top surface of the mattress (e.g., applied by a person resting on the mattress) increases. The void space in channel 30 between the top layer 11 and bottom layer 22 provides a progressive support profile that can simulate a support profile of mattresses using conventional progressive spring coils. As a force is applied to the top surface of foam core 300, the top layer 11 provides a certain degree of support due to the support characteristics of the foam used in the top layer 11. As the top layer 11 compresses and the protrusions 16 of the top layer are pressed downward through the channels 30 and come into contact with the recessions 25 of the bottom layer 22, a greater degree of support is provided due to the support characteristics of the foam of the bottom layer 22 foam combining with the support characteristics of the form of the top layer 11. The effect is a progressive support profile in which the support provided by the foam core 300 increases as more force is applied from the top of the foam core.

Various support characteristics can be achieved by selecting the types and/or densities of foams used for the top and bottom layers and by selecting the distance in the channel 30 between the top layer 11 and the bottom layer 22.

Such a configuration also provides enhanced cooling properties to the progressive foam core 300.

Alternatively, there is substantially no such void space or channel between the most protruded surfaces of the protrusions 23 of the top surface of the bottom layer 22 and corresponding surfaces of the recessions 14 of the bottom surface of the top foam layer 11.

Consistent with an illustrative embodiment, surfaces of the protrusions 23 of the top surface of the bottom layer 22 are adhered to the corresponding surfaces of the recessions 14 of the bottom surface of the top layer 11 by an adhesive. However, no adhesive exists between the most protruded surfaces of the protrusions 16 of the bottom surface of the top layer 11 and the corresponding surfaces of the recessions 25 of the top surface of the bottom layer 22. Using a roller, adhesive is applied only to the top surfaces of the protrusions 23 of the bottom layer 22. The top layer 21 is then positioned over and placed on top of the bottom layer 22. The top surface of the protrusions 23 then adheres to the corresponding surfaces of the recessions 25.

According to the illustrative embodiment shown in FIG. 3A, the height at the protrusions 16 may be 2.0″-5.0″ and the height at the recessions 14 may be 0.75″-3.5.″ Further, the height at the protrusions 23 may be 2.0″-6.0″ and the height at the recessions 25 may be 1.0″-3.0″. The total height of the progressive foam core 300 may be 2.75″-9.5″. As shown in FIG. 3A, the height of the channel 30 is at least 0.25″, but less than 1.5″. There is substantially no space between the protrusions 23 and the recessions 14. However, the present invention is not limited to the above dimensions and one of ordinary skill in the art would recognize that various modifications could be made to the above dimensions consistent with the present invention.

According to the illustrative embodiment shown in FIG. 3A, channels 30 are formed above the top surface of the bottom layer 22. The channels 30 extend at least partially across the width of the bottom layer 22, the width direction extending perpendicular to a head-to-foot length direction of the bottom layer 22. According to an illustrative embodiment, the channels 30 may extend across the entire width of the bottom layer 22.

The channels 30 facilitate airflow across the width of the bottom layer 22. The dimensions and/or configuration of the channels 30 can be adjusted to control and reduce the heat insulating properties of the bottom layer 22 to thereby provide increased comfort to a person resting thereon.

According to an illustrative embodiment, the channels 30 may be wave-shaped. Alternatively, the channels 30 can extend longitudinally, diagonally, or in other directions across the bottom layer 22. As another alternative, the channels 30 may have a shape other than a wave-shape, so long as the channels 30 allow air to flow across the top of the bottom layer 22.

Attached to one or more sides of the mattress is a 3-D fabric (not shown). The 3-D fabric 16 is disposed along the two side edges of the foam core 300 where the ends of the channels 30 are located. As will be understood by those skilled in the art, the 3-D fabric can be a 3-D mesh or a spacer net type fabric. This 3-D fabric has an open knit construction that provides a low pressure environment that allows air to flow easily through the fabric, requiring little force to draw air through the fabric. The 3-D fabric is attached to the sides of the foam core 300.

According to an illustrative embodiment, cool air moves by convection, across the top surface of the top layer 11. When a person lies on a mattress comprising the top layer 11 and the bottom layer 22, air warmed by that person's body rises causing cooler air to be drawn through the holes 17 in the top layer 11 and also through the channels 30. Although heat is typically retained by polyurethane foam, the increased airflow across the top of the mattress provides a cooling effect to the person lying on the mattress. The rising body heat creates a convective air current that draws the cooler air through the plurality of holes 17 in through the channels 30 and the 3D fabric. This cooler air flowing over the top of the top layer 11, operates to cool the body, allowing the person to rest comfortably on top of the mattress without overheating.

Further, according to an illustrative embodiment, the channels 30 may extend so as to connect with vents (not shown) placed on the top or bottom end surfaces, or the side surfaces of the bottom layer 22, either in place of or in addition to the 3D fabric. Such vents may penetrate to the exterior of a mattress. Any number of vents may be used depending on the desired amount of airflow to provide to the bottom layer 22. The vents are small metal or plastic slotted ventilation members that allow air to easily flow through the mattress cover and to the channels 30. Alternatively, other types of vents may be used so long as air passes through the vents with little resistance.

When a person lies on top of a mattress comprising the bottom layer 22, heat from the person's body warms the surrounding air causing it to rise. Through convection from the rising warm air, cooler air is drawn through the holes 17 and through the vents into the channels 30. This air drawn in from the holes and vents travels along the wave-shaped channels cooling the body, resulting in a more comfortable sleep surface for the person.

FIG. 4 illustrates a cross-sectional view of a typical build-up configuration of a progressive foam mattress 400 consistent with an illustrative embodiment.

As shown in FIG. 4, a comfort (or topping) layer 40 may be provided above the top layer 11A. A base layer 45 may also be provided below the bottom layer 22A. Further, a rail 46 may be provided around the perimeter of the progressive foam mattress 400.

According to the illustrative embodiment shown in FIG. 4, the comfort layer 40 may be either flat or convoluted and may be made of foam, for example. The comfort layer 40 may have a height of 2.0″, but is not limited to these dimensions.

As shown in FIG. 4, the base layer 45 may have a height of 1.0″, but the base layer 45 is not limited to these dimensions.

According to the illustrative embodiment shown in FIG. 4, the top layer 11A has protrusions 16A and recessions 14A. The bottom layer 22A has protrusions 23A and recessions 25A. The height of the protrusions 16A may be 3.5″ and the height of the recessions 14A may be 1.75″. The height of the protrusions 23A may be 4.75″ and the height of the recessions 25A may be 2.375″. The total height of the top layer 11A and bottom layer 22A may be 6.5″. The total height of the progressive foam mattress 400 may be 9.5″. However, the present invention is not limited to the specific dimensions of the illustrative embodiment shown in FIG. 4.

FIG. 5 illustrates a cross-sectional view of a firm build-up configuration of a progressive foam mattress 500 consistent with an illustrative embodiment. The illustrative embodiment shown in FIG. 5 provides firmer support characteristics as compared to the illustrative embodiment shown in FIG. 4.

As shown in FIG. 5, a comfort (or topping) layer 40 may be provided above the top layer 11B. A base layer 45 may also be provided below the bottom layer 22B. Further, a rail 46 may be provided around the perimeter of the progressive foam mattress 500. In contrast to the illustrative embodiment shown in FIG. 4, the illustrative embodiment shown in FIG. 5 additionally comprises a high support layer 59. The high support layer 59 may have a height of 2.0″, but is not limited to these dimensions.

According to the illustrative embodiment shown in FIG. 5, the comfort layer 40 may be either flat or convoluted and may be made of foam, for example. The comfort layer 40 may have a height of 2.0″, but is not limited to these dimensions.

As shown in FIG. 5, the base layer 45 may have a height of 1.0″, but the base layer 45 is not limited to these dimensions.

According to the illustrative embodiment shown in FIG. 5, the top layer 11B has protrusions 16B and recessions 14B. The bottom layer 22B has protrusions 23B and recessions 25B. The height of the protrusions 16B may be 2.5″ and the height of the recessions 14B may be 1.0″. The height of the protrusions 23B may be 3.5″ and the height of the recessions 25B may be 1.5″. The total height of the top layer 11B and bottom layer 22B may be 4.5″. The total height of the progressive foam mattress 500 may be 9.5″. However, the present invention is not limited to the specific dimensions of the illustrative embodiment shown in FIG. 5.

FIG. 7 illustrates a flow chart describing a method of manufacturing a progressive foam mattress consistent with an illustrative embodiment. As shown in FIG. 7, operation S100 comprises splitting a first foam member 100 in half to form a first top layer 11 having a series of protrusions 16 and recessions 14 and a second top layer 12 having a series of protrusions 13 and recessions 15.

Operation S200 comprises splitting a second foam member 200 in half to form a first bottom layer 21 having a series of protrusions 26 and recessions 24 and a second bottom layer 22 having a series of protrusions 23 and recessions 25.

Finally, operation S300 comprises positioning the first top layer 11 on top of the second bottom layer 22 so that the most protruded surfaces of protrusions 16 are separated from corresponding surfaces of recessions 25 by a channel 30, and there is no channel between the most protruded surfaces of protrusions 23 and corresponding surfaces of recessions 14.

The foregoing illustrative embodiments and advantages are merely illustrative and are not to be construed as limiting the present invention. It will be understood by those of ordinary skill in the art that the present teachings can be readily applied to other types of apparatuses. Also, the description of the illustrative embodiments is intended to be illustrative, and not to limit the scope of the claims, and many alternatives, modifications, and variations will be apparent to those skilled in the art. 

What is claimed is:
 1. A mattress comprising: a top foam layer; and a bottom foam layer, wherein a bottom surface of the top foam layer comprises a series of protrusions and recessions, wherein a top surface of the bottom foam layer comprises a series of protrusions and recessions, wherein the protrusions of the top surface of the bottom foam layer protrude into the recessions of the bottom surface of the top foam layer, wherein the protrusions of the bottom surface of the top foam layer protrude into the recessions of the top surface of the bottom foam layer, wherein most protruded surfaces of the protrusions of the bottom surface of the top foam layer are separated from corresponding surfaces of the recessions of the top surface of the bottom foam layer by a channel, and wherein there is no channel between most protruded surfaces of the protrusions of the top surface of the bottom foam layer and corresponding surfaces of the recessions of the bottom surface of the top foam layer.
 2. The mattress according to claim 1, wherein the most protruded surfaces of the protrusions of the top surface of the bottom foam layer are adhered to the corresponding surfaces of the recessions of the bottom surface of the top foam layer by an adhesive, and wherein no adhesive exists between the most protruded surfaces of the protrusions of the bottom surface of the top foam layer and the corresponding surfaces of the recessions of the top surface of the bottom foam layer.
 3. The mattress according to claim 1, further comprising first ventilation channels formed in the top surface of the bottom foam layer, wherein the first ventilation channels extend between opposing side end surfaces of the bottom foam layer in a width direction of the mattress, the width direction extending perpendicular to a head-to-foot length direction of the mattress.
 4. The mattress according to claim 1, further comprising second ventilation channels extending in a height direction of the mattress from a top surface of the top foam layer to the bottom surface of the top foam layer.
 5. The mattress according to claim 1, wherein a height of the channel in a direction perpendicular to the top surface of the bottom foam layer is at least 0.25 inches.
 6. The mattress according to claim 5, wherein the height of the channel in the direction perpendicular to the top surface of the bottom foam layer is less than 1.5 inches.
 7. The mattress according to claim 1, wherein the mattress is configured to provide progressively firmer support characteristics as force applied to a top surface of the mattress increases.
 8. A method for manufacturing a mattress, the method comprising: cutting a first foam member in half to form first and second top layers, a bottom surface of the first top layer comprising a series of protrusions and recessions and a top surface of the second top layer comprising a series of protrusions and recessions opposite to the series of protrusions and recessions of the first top layer; cutting a second foam member in half to form first and second bottom layers, a bottom surface of the first bottom layer comprising a series of protrusions and recessions and a top surface of the second bottom layer comprising a series of protrusions and recessions opposite to the series of protrusions and recessions of the first bottom layer; and positioning the first top layer on top of the second bottom layer such that the protrusions of the top surface of the second bottom layer protrude into the recessions of the bottom surface of the first top layer and the protrusions of the bottom surface of the first top layer protrude into the recessions of the top surface of the second bottom layer; wherein the cutting the first foam member in half and the cutting the second foam member in half are performed such that, when the first top layer is positioned on top of the second bottom layer, most protruded surfaces of the protrusions of the bottom surface of the first top layer are separated from corresponding surfaces of the recessions of the top surface of the second bottom layer by a channel, and there is no channel between most protruded surfaces of the protrusions of the top surface of the second bottom layer and corresponding surfaces of the recessions of the bottom surface of the first top layer.
 9. The method according to claim 8, further comprising positioning the second top layer on top of the first bottom layer such that the protrusions of the bottom surface of the first bottom layer protrude into the recessions of the top surface of the second top layer, and the protrusions of the top surface of the second top layer protrude into the recessions of the bottom surface of the first bottom layer, wherein the cutting the first foam member in half and the cutting the second foam member in half are performed such that, when the second top layer is positioned on top of the first bottom layer, most protruded surfaces of the protrusions of the top surface of the second top layer are separated from corresponding surfaces of the recessions of the bottom surface of the first bottom layer by a channel, and there is no channel between most protruded surfaces of the protrusions of the bottom surface of the first bottom layer and corresponding surfaces of the recessions of the top surface of the second top layer. 